In general, drying period of time is determined according to the quantity of either laundry or washing water in a washing tub.
During the drying period of time determined as above, drying is proceeded by rotating a drying tub.
In case the drying period of time is controlled by the quantity of laundry, the quantity of laundry in the washing tub is detected followed by presetting of an appropriate washing water level, washing period of time and drying period of time according to the detected quantity of laundry. Washing is carried out by pulsating washing water with a washing motor for the preset period of washing time, and, on finish of washing, drying is proceeded by rotating the drying tub for the preset drying period of time.
To detect the quantity of laundry in the washing tub, power supply to the washing motor is cut-off after running the washing motor for a certain period of time. When power supply to the washing motor is cut-off, the washing motor does not stop at once but slows down by the inertia until it comes to a complete stop. The washing motor stops earlier when there is more laundry due to the friction between laundry and a pulsator. Number of waves of remaining voltage generated by the inertia of the washing motor is detected so as to determine the quantity of laundry in accordance with the number of waves transmitted. Based on this determination, presettings of the washing water level, the washing period of time and the drying period of time are carried out.
However, because the presettings of the washing and the drying period of time only through the detection of quantity of laundry is not sufficient enough, an adjustment of the drying period of time according to the kind of laundry is carried out. As the judgement on the kind of laundry by users may not be precise in most cases, it has been a problem that the unrequired drying time was wasted accordingly. Particularly, the drying period of time for laundry of easy drying can, most probably, be preset longer than required.
Further, the determination of the quantity of laundry in the washing machine has to be carried out in comparison with the data memory which is obtained through experiments of drying period of time according to the quantity of laundry, of which setting of experimental criteria and exact calculation of data are not simple, and production cost is high due to the great number of memory.
On the other hand, in case the drying period of time is controlled by the quantity of washing water, the washing water in a drying tub is passed with light, with which quantity of passed light the drying period of time is preset.
However, this technology also has problems that, because the drying period of time is controlled, not taking into account of the quantity and kind of laundry, but comparing the passed quantity of light with a certain criteria, an excessive drying over the appropriate drying period of time, occurs when the quantity of laundry is much, and an appropriate presetting of drying period of time can not be obtained when the quantity of laundry is small because the period of time reaching to a certain criteria is too fast. Moreover, as the degree of water soaking and draining to/from laundry is different depending on the kind of laundry, there has been a problem that the most suitable control of drying period of time is hard to obtain with this type of control.